Solid propellants containing burning rate depressants

ABSTRACT

This patent described novel solid ammonium perchlorate containing propellant compositions containing powerful burningrate depressants which are potential bases, that is, compounds which are capable of releasing free bases. The burning-rate depressants of this invention do not reduce the specific impulse of the propellant, nor do they interfere with cure reactions or adversely affect the physical properties of the cured propellant.

United States Patent Inventors Adolf E. Oherth Fair Oaks; Roll 8. Bruenner, Orangevnle, both of Calif. Appl. No. 726,649 Filed May 2,1968 Patented Dec. 7, 1971 Assignee Aerojet-General Corporation El Monte, Calif.

SOLID PROPELLANTS CONTAINING BURNING RATE DEPRESSANTS 12 Claims, 10 Drawing Figs.

U.S. Cl 149/19, 149/20 Int. Cl C06d 5/06 Field of Search 149/19, 20

References Cited UNITED STATES PATENTS 3/1962 Sammons 149/76 X Primary Examiner-Benjamin R. Padgett Atrorneys- Edward O, Ansell, D1 Gordon Angus and Reid Anderson ABSTRACT: This patent described novel solid ammonium perchlorate containing propellant compositions containing powerful burning-rate depressants which are potential bases. that is. compounds which are capable of releasing free bases. The burning-rate depressants of this invention do not reduce the specific impulse of the propellant, nor do they interfere with cure reactions or adversely affect the physical properties of the cured propellant.

SOLID PROPELLANTS CONTAINING BURNING RATE DEPRESSANTS BACKGROUND OF THE INVENTION The field of the present invention pertains to novel solid rocket propellants and their preparation.

Solid rocket propellants employing ammonium perchlorate as an oxidizing agent are well known. In general, propellants based upon ammonium perchlorate have a high burning rate. In many solid rocket applications, it is desirable to have a propellant which will provide a slower burning rate than normally can be had with ammonium perchlorate propellants. Therefore, it has been proposed to use various burning-rate depressants to control and reduce the burning rate of ammonium perchlorate propellants. Typical of such known bumingrate depressants is calcium oxalate. However, it is necessary to use calcium oxalate in rather substantial quantities if the desired burning-rate suppression is to be obtained. Moreover, calcium oxalate does not contribute to the energy of the propellant and hence, when used in effective buming-rate depressing amounts, reduces the specific impulse of the propellant.

According to the present invention there has now been found potential bases which are uniquely adapted to the reduction of the burning rate of ammonium perchlorate containing propellants. These additives are substantially more ef fective than calcium oxalate in that they require only about one-tenth or less of the concentration of calcium oxalate in order to provide the same degree of buming-rate suppression. In addition, the buming-rate additives of the present invention do not decrease the propellant's specific impulse.

SUMMARY OF THE INVENTION Briefly, the present invention comprises novel solid ammonium perchlorate containing rocket propellants containing therein potential bases which are stable under conventional solid propellant curing conditions but which yield at least some free base upon propellant combustion. The amine precursors reduce the burning rate of the propellant without adversely affecting the specific impulse or the physical properties thereof. The invention further comprises the formulation of novel propellants containing the burning-rate depressants referred to above.

The precursors are used in an effective burning rate reducing amount, normally from about 0.0l percent to about 5 percent by weight of the propellant. The optimum amount for any propellant is dependent upon the propellant formulation and performance desired.

While not bound by any theory, it is believed that the novel burning-rate depressants of the present invention act as sources of free nonvolatile bases such as amines, that is, compounds which release bases upon thermal decomposition only. It has been found that nonvolatile free organic amines in a propellant binder drastically reduce the burning rate, apparently due either to the generation of ammonia or to the prevention of the buildup of a hydrogen-perchlorate-rich surface on the NH,CIO, particles. However, the incorporation of strongly basic free amines in a propellant is impractical, since it was found that the ordinary binder systems used in the solid propellants will not satisfactorily cure in their presence. The precursor compounds of the present invention are believed to generate the free nonvolatile amines in situ, and therefore permit the curing of the propellant and at the same time yield, upon decomposition, the free nonvolatile amines which suppress the buildup of perchloric-acid-rich surfaces on the NH,C10, particles.

The control of the ammonia generation and perchloric acid buildup is believed to be significant for the following reasons. It is well known that in an ammonia atmosphere, the decomposition of ammonium perchlorate is considerably slowed down. It is believed that the decomposition of ammonium perchlorate is preceded by a dissociation into ammonia and hydrogen perchlorate, followed by desorption where ammonia, because of its lower molecular weight, diffuses away faster, leaving a hydrogen-perchlorate-rich surface. In the present invention, it is believed that the burning-rate depressants of this invention prevent temporarily the buildup of a hydrogen-perchlorate-rich surface. The compounds of the present invention are very beneficial in controlling the burning rate of ammonium perchlorate propellants. In addition, the compounds of the present invention are fully compatible with the binder system and do not interfere with curing. These compounds are also sufficiently soluble in the binder to allow themselves to be adsorbed onto the ammonium perchlorate.

Accordingly, it is an object of the present invention to provide a novel class of solid rocket propellants.

More particularly, it is an object of the present invention to provide a novel class of solid rocket propellants having a decreased burning rate and containing precursors of free organic amines.

Yet another object of the present invention is to provide the means for controlling the burning rate of ammonium perchlorate propellants without adversely affecting the specific impulse.

Still another object of the present invention is the provision of buming-rate modifier compounds which are effective in ammonium perchlorate propellants and do not interfere with the cure system or the physical properties of the cured propellant.

These and other objects and advantages of the present invention will become apparent from the detailed description which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS New classes of buming-rate modifiers within the scope of the present invention are those compounds derived from amines which will yield free amines upon thermal decomposition or those which are capable of exchanging the amine constituent. The present invention is not limited to amine derivatives, however, but extends to compounds which are potential bases, that is, compounds or compositions which, at elevated temperatures, are converted into or release free bases and are readily adsorbed or coated onto the oxidizer (ammonium perchlorate or other ammonium salts with an oxidizing anion like NO included are also salts of ammonium derivatives, such as hydroxyl-ammonium The amine derivatives within the scope of the present invention include ammonium salts of organic amines having the formula:

wherein R, and R, are either alkyl groups or hydrogen, and R; is preferably a hydrocarbon chain containing from eight to 30 carbon atoms. Any of the R-groups may contain one or more functional groups such as hydroxyl, etc. Examples of acid used to prepare the salts from the above amines include hydrochloric, sulfuric, nitric, perchloric, hydrogen borofluoride and perfluoroaeetic.

The invention also includes quaternary ammonium salts having the formula:

Iitl Rr-N-Rr wherein R,, R,, R and R, are alkyl groups, preferably where at least one R group is a hydrocarbon chain containing from about eight to about 30 carbon atoms. Any of the R-groups may contain one or more functional groups such as hydroxyl,

carboxyl, etc. X is any of the stable inorganic or organic anions, i.e., chloride, nitrate, sulfate, phosphate, etc. Included are also quaternary ammonium salts which are derived from oligoand polyamines.

An example of the preparation of quaternary ammonium salts 1936) trimethylhexadecylammonium iodide wherein 13.5 g. dimethylhexadecylamine are dissolved in approximately 100 ml. methylethylketone (MEK) and then 7 g. methyliodide added under stirring. This mixture is heated to the boiling point of MEK and isopropanol is added until all of the precipitated quaternary ammonium salt has dissolved. The clear solution is allowed to cool to room temperature whereupon the salt crystallizes out. After washing with MEK the compound was dried under vacuum. White crystalline solid, melting point close to that reported in the literature (247 C.) (Macroski, Bulletin de la Societe Chimique de France (5) 3 I936) 500); yield is nearly quantitative.

Another example of the preparation of quaternary ammonium salts is trimethyldodecylammonium toluenesulfonate wherein l0 g. dimethyldodecylamine are dissolved in about 50 ml. methylethylketone and 9.3 g. methyl p-toluenesulfonate added under stirring. The quaternary ammonium salt starts to crystallize out already at room temperature. To complete the reaction the solution is heated to the b.p. of MEK and just enough isopropanol added to completely dissolve any precipitate. The desired compound crystallizes out on cooling to room temperature. It is filtered off, washed with MEK and dried under vacuum. White, crystalline solid; yield nearly quantitative. 30

A still further example of the preparation of quaternary ammonium salts is trimethyldodecylammonium metaphosphate wherein a cold solution of L2 g. water-soluble sodium metaphosphate in 100 ml. water, a hot solution of 4 g. trimethyldodecylammonium toluenesulfonate in about 30 ml. 35

water is added under rapid stirring, whereupon the quaternary ammonium metaphosphate precipitates immediately. It is filtered off, washed with plenty of cold water and dried under vacuum. White solid, soluble inorganic solvents like Cl-lCl, and others.

The following quaternary ammonium halide compounds have also been found to be useful in this invention.

Other quaternary ammonium salts useful in this Invention and containing hydroxyl groups include:

a lit e R0 R1 X (11).. lliUlhUll (llhClhOll *GIIsClliOil CU; I (11).... blamensnbovuuuflhn. lnmnasnbovondflnfl CH; CH; 1 (c) (Ill; "(1H CH; Clh PH Cll; Cull Cll, 0 80(311,

(d) "Cl iClI'CH' -C|5Hn -CIII -CIX; Cl

0 ll ll (0) Same as above CH:CH:0II CII;CHOH CnH;1 C] (i) -CH1CH20H C 2Hzs *CH; 01 Same as above CuHn CH| Cl .do .d0 Same as above -C|5H Br *CII --CH-CH; CH:7-CH*-C1{3 0 SOCH;

H OH

(j) *CHz-CII-CHI CH:CH:OH *CHzCHgOH C1 H;1 Cl

H II

Same as above -C1sHn CH: CH; Cl -CH2CH1OH CH:CH2OH "CH; CH: I Same as above "do CHzCIhOH CH; I "CHHH --C2Ii5 CII3 CH3 H;C v S O;

(o) Same as above *CH; CH; -CH! Same as above. (9)" (10 CH; *Clla CH; DD. i H; -CH2CHOH -CH2CH2OH Do. (r) *CHICIIECN Same as above Same as above Do. CH; Cli -CH1-CHCH, Do.

NCCH2CHF 'CIhCIIflOH -CH1CHaOH Do. -CH; --CII; CH;.CHOH.CH;OH Do. --ClhClIOlI CH; -CH .CH0H.CH, "'CllQ-(IIIOIIXJH; D0. H; C IzClh0H --CH1CII2OH Do. CH3 -C H3 (1121175 D0. 015F131 *CHsC 20H ACHQCIIIOI D0. CH: Cla aT lE JT 0- Polymeric quaternary ammonium salts with OH groups are also included, and are illustrated by:

wherein R,, R and R are alkyl groups; R, is any linear or branched hydrocarbon group, normally alkylene; and Y may be any of the stable inorganic or organic groups such as OBF,, S and COO. In the above fonnula, the R,-group can be omitted in which case the Y-group (BF,, 0, etc.) is attached directly to the nitrogen. In the above fonnula, any of the R-groups may comprise a hydrocarbon structure from eight to 30 carbon atoms. Also, any of the R-groups may contain one or more functional groups such as hydroxyl, carboxyl, etc. Included also are structures which contain more than one functional group such as hydroxyl, carboxyl, etc. Included also are structures which contain more than one anionic Y-group.

An example for the preparation of a Betaine Analogue having the formula:

(according to T. L. Gresham et al., J.A.C.S. 73, 3,168, I95 1 To a solution of 2| .3 g. dimethyldodecylamin in 125 ml. ether and 38 ml. H,CCN 7.2 g. B-propiolactone were added under stirring, whereupon a white solid formed. After standing for 16 hours, the solid was filtered off and washed with ether. The compound was dried under vacuum and kept over I50 yield 24 g. (81 percent).

An example for the preparation of a tertiary amine-BF, adduct having the formula:

wherein 0.8 moles of dry ether were placed in a three-neck flask and BF, was passed in through a glass-fritted tube until vapors of BF, could be detected above the solution; some more ether was then added to convert all excess BF, to the BF etherate. To this solution 0.8 moles of dimethyldodecylamine were added under stirring. The ether was then removed under aspirator vacuum. After this, all remaining BF -etherate was distilled off under oil pump vacuum at 80 C. The residue crystallized at room temperature and was recrystallized from heptane; white flakes.

An example for the preparation of an oxyfluoborobetaine having the formula:

(according to H. Meerwein et al., J. Prakt. Chem. (2) 154,83 (i940)), wherein 0.25 moles of and 0.25 moles of dodeceneoxide were heated under stirring in a C. oil bath. At 70-75 C. the reaction starts with an exothenn, which takes the mixture of about llS-l20 C. At the same time the color changes from a pale yellow to the color of honey. The compound is a rather viscous liquid at 90 C. and vaselinelike at room temperature.

An example for the preparation of a sulfobetaine (according to J. H. Helberger, Ann. 588, H (1954)) wherein 6. I07 g. y-propanesultone and 10.65 g. dimethyldodecylamine are heated in about 50 ml. xylene. At -9$C. an exothermic reaction starts under precipitation of the desired compound. The mixture is taken to ca. C. and held there for 10 mins. to complete the reaction. After cooling to room temperature the solvent is removed by filtering; The residue is stirred with acetone (Which facilitates filtering), filtered and washed several times with acetone. The compound is then dried under vacuum at about 60-70 C.; white solid.

Another class of compounds within the scope of the present invention are the substituted ureas having the formula:

wherein R is a hydrocarbon group, and R R and R are hydrogen or hydrocarbon groups, preferably with one or more of the R-groups consisting of hydrocarbon chains from about eight to about 30 carbon atoms. Any of the R-groups may contain one or more functional groups such as hydroxyl, carboxyl, etc. Also included are derivatives of biuret, guanidine and similar compounds.

The following is an example for the preparation of a substituted urea having the formula [(-CH1)NH-CO-NH- C.-,Hz:,]2:l8.5 g. dodecylamine and 8.4 g. hexamethylene diisocyanate are dissolved separately in heptane and then the isocyanate solution is added to the amine solution with good stirring. The product precipitates out immediately. It is filtered off and washed thoroughly with heptane; white solid.

Still further, the invention includes any of the metal salts which form stable complexes with amines. The amino nitrogen can carry at least one hydrocarbon group with or without other functional or complexing groups. Preferably the hydrocarbon group contains from about eight to 30 carbon atoms. An example of such a compound is the zinc complex:

The preparation of the potential base compounds is not new, and the general methods can be found in e.g., Houben Weyl, Methoden der organischen Chemie, Vol. XI/Z, Stuttgart Georg Thieme Verlag, 1958, pp. 59l-6l3 (for quaternary ammonium compo including betaines and betaine unds analogues), Vol. VIII (1952), pp. l57-l58 (for substituted ureas), pp. 180-l83 (for substituted guanidinium salts), Vol. III (1958 pp. 39l-4 ID (for urea clathrates The synthesis of sulfobetaines can be found in: R. F. Fischer, Ind. & Eng. Chem. Vol. 56, No. 3, p. 44 I964) etc.

The following examples are presented solely to illustrate the invention and should not be regarded as limiting in any way. In the examples, the parts and percentages are by weight unless otherwise indicated.

Amine precursor compounds were evaluated, by incorporation at an 0.3 percent level into a propellant consisting of 86 percent solids [76 percent ammonium perchlorate, and I0 percent powdered aluminum). The additive replaced an equivalent amount of the binder. Direct addition of the respective amine precursor compound to the binder is preferred, though in special cases the oxidizer was precoated with the material. Processing was done at ll5-l25" F. The propellants were cured at I35 F. in bulk for preparation of solid strands. Since the response of a propellant towards such bumingrate modifiers depends strongly on the binder, two representative binders were used for testing.

Binder No. l

Polypropylene ether diol 28.435 Polypropylene ether triol 4| 10% llodecylpelsrgonst: 25 00% Hes-methylene diisocyannte (I07 eqsIlOOeqs ON) 5475 N-phenyl belannphtliylsmine 011i Catalyst is either FeAA (0.015-003 percent) or dibutyltindilaurate (0.05 percent).

Binder No. 2

Carbosy-terminated polybutadiene (IOOeqs) sans Trimethyllziridine phosphine oxide (I20 eql] 2.07% lsodecylpelargonate 30.0%

The following compounds are a few examples of those synthesized and tested:

In the Binder No. 1 system (FIG. I) it is seen that the 3) (f) is more effective than the iodide (3) of an identical cation. If compared with the iodide series [RN(CII,),] I when R I) C,,H, and (2) C H then compound (e) R=C,,,l-I, is the least effective, whereas (2) R==C,,l-l is the most effective. The mole concentrations of these compounds are 2.82, 2.43 and L48 respectively. The effectiveness passes through a maximum. The bromides (b) and (c) (FIG. 2) (Binder No. l)compare favorably with any of the other compounds.

Sulfobetaines of the following formula were used in Binder No. l (FIG. 3), and Binder No. 12 (FIG. 4):

In addition to the two compounds with R,=C,,I-I,,, and R,=C,,,H and R,=R,=-CH;,R,( CH, a compound with R,=C,,,H was prepared, which proved to be the most effective one in both systems (FIGS. 3 and 4).

Ureas of the formula:

WITH R and R hydrocarbon was used.

The following compounds were prepared and tested:

The results are shown in FIGS. 5-8. In both systems. Binder No. I (FIGS. 5 and 6) and Binder No. 2 (FIGS. 7 and 8), it is seen that these compounds indeed are burning rate modifiers though not as effective as the previous ones.

The following compound was prepared according to Ralston et al. .I. AM. Oil Chemists Soc., 24, page 307:

Mixtures of the above compound with AP (CD-blend) showed final decomposition DTA exotherm at 355 C.

One of the most eflective burning-rate depressants reported in the prior art is oxamide. Oxamide was incorporated into standard propellants at a l.5 level and compared with the ad ditives of this invention at an 0.3 percent level. The results are shown in FIGS. 9 (Binder No. 2) and 10 (Binder No. I). For the Binder No. 2 system the quaternary ammonium salt is more than five times as effective as the oxamide. In the Binder No. 1 system, the sulfobetaine shown is even more effective.

The scope of the invention is not limited to amine derivatives; actually any potential base as defined there will be effective although the most powerful burning-rate depressants are derived from amines. An example for this are depressants derived from carboxylic acid salts, preferably from such which are relatively strong acids (e.g., by a-substitution with electron withdrawing groups):

wherein R alkyl or aryl (preferably long hydrocarbon chains, e.g., C I-I, and longer); R, preferably -OH; R,=I'I or aryl and Y preferably cations of the alkali or alkaline earth metals, like Li, Na, K, Mg, Ca Sr and Ba.

A example for the preparation of a carboxylic acid salt is Libenzilate wherein 0.05 moles of benu'lic acid are dissolved in warm methanol and a solution of 1.2 g. LiOH (in just enough methanol to dissolve it) is added. The Li-salt crystallizes out at room temperature and can be recrystallized from isopropanol. (Benzilic acid can be either purchased or made according to "Organic Synthesis" Collective Vol. I, p. 89 (1958 publ. John Wiley & Sons, Inc., New York.

As used herein, potential base means that free bases will not only be released from real chemical compounds, but also from adsorbates, clathrates (inclusion compounds) etc. An example for the latter are amine/urea inclusion compounds, where the amine is contained in channel structures formed by the urea crystal lattice. At the m.p. of the urea (about 130 C.), the amine is released and is free to difl'use through the binder to the NI-LC I 0, particles.

An example for the preparation of an alkylamine ureaclathrate is where 40 g. of urea are dissolved in I00 ml. 10.4 methanol, then I04 3. dimethyloctadecylamine are added under stirring; the clathrate crystallizes out on cooling under continued stirring. It is filtered off at room temperature, washed with ice cold methanol and dried under vacuum at room temperature. Titration of the clathrate showed that it contained 75 moles amine per g. clathrate or 22.3 g. amine per 100 g. clathrate.

The present invention is applicable to all types of ammonium perchlorate containing solid propellants including those having polymeric binders derived from polyether-diols, -trio|s etc., and polybutadiene-diols, -triols etc. with isocyanates as curatives (polyurethanes), or derived from polybutadienedicarboxylic acids and aziridines or aziridine derivatives as curatives. The propellants may contain fuels such as aluminum, plasticizers, stabilizers and the like. Many propellant formulations and their processing techniques are disclosed in assignees prior patents including US. Pat. Nos. 3,245,849 and 3,340,] l l the disclosure of which are expressly incorporated herein by reference.

Having fully described the invention, it is intended that it be limited only by the lawful scope of the appended claims.

We claim:

I. A novel solid ammonium perchlorate containing rocket propellant composition containing a polymeric organic binder and an effective buming-rate depressing amount of a potential base which is stable under normal solid propellant curing conditions but which upon combustion of the propellant undergoes conversion to form at least some free base, said potential base being an organic nitrogen compound and present in an amount from about 0.0l percent to about 5 percent by weight of the propellant.

2. In solid rocket propellants comprising a cross-linked high molecular weight polymeric binder and an ammonium perchlorate oxidizing agent, the improvement wherein said propellant contains an effective burning-rate depressing amount of a potential base which is stable under the normal curing conditions for solid rocket propellants but which is capable upon combustion of said propellant of conversion to form at least some free base, said potential base being an organic nitrogen compound and present in an amount from about 0.01 percent to about percent by weight of the propelan ammonium salt of an organic amine.

8. The composition of claim I wherein the potential base is a quaternary ammonium salt.

9. The composition of claim 1 wherein the potential base is a zwitterionic betaine-type compound.

10. The composition of claim 1 wherein the potential base comprises a substituted organic urea.

IL A method of fonning a novel solid rocket propellant comprises forming a castabie mixture of a curable polymeric binder, ammonium perchlorate, and a potential base, and thereafter casting the mixture and permitting it to set to form a solid rocket propellant having a depressed burning rate, said potential base being an organic nitrogen compound and present in an amount from about 0.01 percent to about 5 percent by weight of the propellant.

12. The method of claim ll wherein the potential base is an organic amine precursor.

i i I U 0 

2. In solid rocket propellants comprising a cross-linked high molecular weight polymeric binder and an ammonium perchlorate oxidizing agent, the improvement wherein said propellant contains an effective burning-rate depressing amount of a potential base which is stable under the normal curing conditions for solid rocket propellants but which is capable upon combustion of said propellant of conversion to form at least some free base, said potential base being an organic nitrogen compound and present in an amount from about 0.01 pErcent to about 5 percent by weight of the propellant.
 3. The composition of claim 1 wherein the propellant additionally contains a finely divided metal fuel.
 4. The composition of claim 1 wherein the propellant contains a polyurethane binder.
 5. The composition of claim 1 wherein the propellant contains a binder comprising a carboxy-terminated polydiolefin cross-linked with aziridines or their derivatives respectively.
 6. The composition of claim 1 wherein the potential base is an organic amine precursor.
 7. The composition of claim 1 wherein the potential base is an ammonium salt of an organic amine.
 8. The composition of claim 1 wherein the potential base is a quaternary ammonium salt.
 9. The composition of claim 1 wherein the potential base is a zwitterionic betaine-type compound.
 10. The composition of claim 1 wherein the potential base comprises a substituted organic urea.
 11. A method of forming a novel solid rocket propellant comprises forming a castable mixture of a curable polymeric binder, ammonium perchlorate, and a potential base, and thereafter casting the mixture and permitting it to set to form a solid rocket propellant having a depressed burning rate, said potential base being an organic nitrogen compound and present in an amount from about 0.01 percent to about 5 percent by weight of the propellant.
 12. The method of claim 11 wherein the potential base is an organic amine precursor. 